Investigators purchased 400 Apple chargers from different retailers around the world over the internet and put them through the tests that they would need to pass in order to be sold in shops in the UK. In all, 397 chargers failed.

“Criminals across the globe are using online platforms to lure you in with cheap deals for fake items, many of which are dangerous and have been known to overheat and cause house fires,” said Lord Toby Harris, chairman of Trading Standards, in a statement.

Even seemingly reputable retailers might not always be safe. In October, Apple filed a lawsuit against online sellers, after it was revealed 90 percent of iPhone chargers sold on Amazon were fake.

If you have an iPhone charging cable you bought online, you can use CNET’s handy guide to figure out whether or not it is safe. Apple also provides its own guide to help you spot bogus Lightning connectors.

To be sure you’re buying a bona fide Apple charger, it’s best to buy directly from the company or another reliable retailer.